What is witnessed today is not only an ever-increasing demand for systems producing cold temperatures at cryogenic level but improved performance and good economy of these systems, including their refrigerating capacity, power requirements, operational dependability and some other factors. The high demand seems to be quite natural in the light of the rapid progress made in the field of scientific and applied research work dealing with superconductivity such as the development of electrical machinery, powerful magnets, power transmission lines, electronic devices; the use of liquid hydrogen on a large scale is another source of this demand. Since the equipment employing the phenomenon of superconductivity is bound to operate at temperature as low as 1.5.degree. to 15.degree. K., the power requirements of a cryogenic system used to maintain such temperature can run into hundreds and thousands of kilowatts if the plant cooled is a big one. This all calls for giving top priority to the problem of reducing the amount of energy consumed in producing cold temperature and of improving cryogenic systems so as to increase their operational dependability, reduce weight and size, etc. Not infrequent are such cases when there is a need to lower the level of the cold temperature produced maintaining the economy of the process at a high level at the same time.
What is called actual power requirements in producing cold temperature are given as the ratio of energy consumed, mainly in driving the compressor, to the unit energy removed by the refrigerant in terms of the refrigerating capacity. Since both these values are commonly given in watts, the actual power requirements are expressed by a dimensionless figure (W/W). For those who are versed in cryogenics, the term refrigerating capacity denotes the amount of heat removed by a system per unit time at the temperature level specified.
There is known a method of producing cold temperature involving a number of operations described hereinafter in describing by way of example a helium cryogenic system capable of producing ultra-low temperature at the level of the boiling point of liquid helium, i.e., between 4.2.degree. and 4.5.degree. K.
Gaseous helium is compressed to a pressure of 20-30 bars, using a compressor. The compressed helium constitutes an incoming stream heading toward a refrigerative load, and this incoming stream is cooled to approximately 100.degree. K. by a return stream of helium flowing back from the refrigerative load under a low pressure. After that, the incoming stream in split into two streams of which one is a main stream and the other is a subsidiary stream. This latter stream is expanded in expanders, some external work being done, and is used to compensate for irreversible losses and to cool the main stream in a stepwise manner, the number of cooling stages being determined by the number of expanders used in expanding the subsidiary stream. A bath with a liquid refrigerant, for example, nitrogen or any other substance whose boiling point is sufficiently low to make for the process of cooling is used sometimes instead of the expanders. The main stream, on passing through all the cooling stages, is admitted into a liquefaction stage wherein it is given additional cooling and expanded with liquefaction. The liquid helium formed is used to sustain a refrigerative load and, on absorbing heat therefrom, evaporates. The vapour constitutes a return stream fed back into the liquefaction stage at a temperature of 4.3.degree. to 4.5.degree. K. in a countercurrent flow to the main and subsidiary streams, is warmed up in the heat exchangers of all stages, merged underway with the subsidiary stream expanded in the expanders and admitted into the compressor under the atmospheric pressure and at a temperature of 300.degree. K. for compression. This completes the cycle which is then repeated.
In one version of the method described, the process of expanding the main stream with its liquefaction in the liquefaction stage is accomplished by throttling and in another version the same goal is attained by expansion accompanied by external work. Throttling is practiced for many years, and the process of expansion into the region of saturated vapour accompanied by external work is described in "Cryogenic Engineering" by R. B. Scott, D. Van Nostrand Co. Inc., Princeton, 1959. Considering by way of example the operation of a helium cryogenic system, the author points out the advantages of said process of expansion when this process is being compared with the process of throttling.
In the method of obtaining refrigeration which is now under consideration, the energy consumed in compressing gas is removed not only in order to produce supercold temperature but also serves to compensate for various losses, being consequently dissipated without any useful effect and increasing the entropy of helium. These losses are termed as those due to the irreversibility of the process and they are incurred owing to heat transfer at temperature gradients other than zero, to friction opposing the flow of helium and due to some other factors.
Thermodynamically justified are less than 20% of the energy consumed in cryogenic systems, the balance being wasted in compensating for irreversible losses, the bulk whereof is incurred due to a difference in temperature, particularly in the region of ultra-low temperatures. Calculations reveal that the losses incurred in the last cooling stage of the liquefaction stage are approximately at balance with the net effect, i.e., refrigerating capacity, and any reduction of these losses is conducive to an increase in the efficiency of the method of obtaining refrigeration or in the performance of the system employing same. The effect of the losses of energy due to the irreversibility of the process of, say, heat exchange is that transferred to the incoming stream is only a fraction of the refrigeration the return stream is capable of, the rest being wasted in the course of incomplete heat exchange resulting in an increase of the entropy of helium. The consequence is that less liquid fluid fed to sustain the refrigerative load is formed from the incoming stream, implying that the refrigerating capacity of the system is not as high as this is desirable. On the other hand, an effort to obtain a sufficient amount of liquid fluid without reducing the losses of energy due to irreversibility calls for increasing the amount of energy consumed because more gas should be compressed in the compressor. The losses due to the irreversibility of the process of heat exchange increase with the ratio of the difference in temperatures to the absolute temperature.
Substantial losses due to the irreversibility of the process of heat exchange are incurred in the liquefaction stage even then when the main stream is expanded with liquefaction and external work being done at the same time. Said disadvantage, pointed out in the above work by R. B. Scott, is inherent due to a considerable difference between the temperatures of the incoming and return streams in the liquefaction stage even in a theoretically idealized case, this difference being especially pronounced in the region of the lowermost temperature of the streams and inviting an increase in entropy. The efficiency of the heat exchanger has no bearing on said difference of temperatures which will be present in a theoretically idealized case, i.e., one when the difference between the temperatures of the streams at the cold end of the heat exchanger is equal to zero. When used as the fluid is helium, the difference between the temperatures of the incoming stream, or the main stream as in the case under consideration, and the return stream is around 1.5.degree. K. at the warm end of the heat exchanger in the liquefaction stage, provided the pressure of the incoming stream is 25 bars and that of the return one is 1.3 bar and the temperature of this return stream is 4.5.degree. K. Said temperature difference increases to 2.5.degree. K. toward the medial part of the heat exchanger and then gradually decreases to under 0.5.degree. K. at the cold end of the heat exchanger. The temperature difference of under 0.5.degree. K. is to be regarded as the maximum allowable one among those temperature differences which are compatible with high efficiency of obtaining refrigeration. The direct effect of this diadvantage is an inefficient use of the refrigeration the return stream is capable of over the range of temperatures between the boiling points which is 4.5.degree. K. for helium and the temperature of the compressed stream before its expansion with liquefaction which is around 6.degree. K. for helium. This fact either reduces the refrigerating capacity or increases the amount of energy consumed. If in realizing the known method a specified refrigerating capacity is to be attained, the only answer is to increase the amount of gas compressed which entails a high consumption of energy.
Recent years saw a method of obtaining refrigeration at a cryogenic level between 1.8.degree. and 4.0.degree. K. coming into wide-spread use because, as the research work carried out in vatious countries has proved it, in certain fields as, of example, radio engineering and nucleonics the lowering of the level of the refrigeration obtained by as little as 0.5.degree. K. leads to qualitively new results. Yet, difficulties are encountered in employing cryogenic systems operating at temperature levels below the boiling point of helium under the atmospheric pressure and these difficulties are the soaring costs whenever an attempt is made to obtain a more deeper refrigeration.
Known in the art is a method of obtaining refrigeration at a temperature under 4.0.degree. K. and, in particular, one at the level of 1.8.degree. K. (cf. Ketheder H., Lehmann W., Spath F., "Long-term experience with the liquefying stage and a 4.4.degree. K.-cooling cycle of a 300 W-refrigerator", Proceedings of the Fifth International Cryogenic Engineering Conference, p. 546, Kyoto, 1974, IPC Business Press Ltd., London, 1974) wherein helium is compressed to a pressure of around 20 bars at ambient temperature. The incoming stream of compressed helium so formed is fed to sustain a refrigerative load on being cooled in a stepwise manner by a return stream of the same gaseous fluid flowing back from the refrigerative load in the reverse direction. The cooling of the incoming stream takes place in successively arranged cooling stages by analogy with the method described above.
The fundamental difference of this method compared with the one described hereinabove consists in that the stream of compressed gas is being expanded in the liquefaction stage with its simultaneous liquefaction to a pressure which is below the atmospheric and depends on the level of refrigeration required to obtain. So, if a temperature at a level of 1.8.degree. to 2.0.degree. K. is required, the gas is expanded to a pressure between 12 and 20 mm Hg. Under the same pressure progressing back for the refrigerative load is the vapour of helium constituting the return stream which, on passing through all the cooling stages in the reverse direction, warms up to a temperature close to the ambient and is admitted into a vacuum pump wherein it is compressed to the atmospheric pressure. The outflow from the vacuum pump is fed into the compressor. To be exact, the pressure upstream of the vacuum pump is less than the pressure expanded whereto has been the gas, i.e., anywhere between 12 and 20 mm Hg, owing to the resistance of the heat exchangers to the flow of the return stream.
Should a need arise to feed liquid helium for sustaining a refrigerative load under a pressure in excess of 1 bar and at a temperature below 4.0.degree. K., it can be met by providing an intermediate refrigerative load. Serving as such in this case can the main stream itself which has been expanded in the liquefaction stage to a pressure sustained whereat is the refrigerative load and which needs further cooling. This cooling is effected by way of heat exchange between the main stream and that portion of this stream which has been expanded and is boiling under vacuum.
Since the bulk of the fluid in circulation is subject to additional compression in the vacuum pump, the power requirements this method calls for rise very sharply. For all its ability to offer refrigeration at a level below 4.0.degree. K. and even as low as 1.8.degree. K., this method displays a number of other disadvantages along with high power requirements. Among the disadvantages is the need in intricate and costly equipment, such as vacuum pumps. The heat exchangers used in conjunction with a system realizing this method are, in their turn, bulky and of intricate construction due to the fact that the vapour of helium returns from the refrigerative load under a pressure by far lower than the atmospheric.